Our laboratory has isolated and characterized a proteinase from human and animal skin. We have shown that rat and mouse have similar chymotryptic activities; these enzymes appear to be chymase. By contrast, human proteinase is distinct from the animal sources. The human proteinase is not Cathepsin G. Studies of involved and uninvolved psoriatic skin demonstrate that involved psoriatic tissue has increased levels of plasminogen activator. After grafting of involved and uninvolved skin to nude mice, the increase in plasminogen activator is maintained in involved skin. Interestingly, uninvolved skin demonstrates an increase in plasminogen activator to the level of involved skin after residence on a nude mouse for 4 weeks. The level of plasminogen activator in normal human skin does not change after grafting to the nude mouse. These data suggest that elevations of proteinase activity in involved and uninvolved psoriatic skin are intrinsic to the disease and excellent markers for disease activity.